Previous research has suggested that earlier intervention may contribute to better outcomes in autism. Answering the question how early? has significant health and educational implications for age of screening for autism spectrum disorders (ASD) and age of entry into early intervention for children with ASD. With the increased number of young children identified with ASD, there is a pressing need for evidence-based, manualized intervention that is accessible to and implemented by parents within their natural environments. This project is collaborative with two identical, linked applications submitted by Dr. Amy Wetherby at FSU and Dr. Catherine Lord at UM. The major objective is to directly compare two parent intervention conditions for 9 months each: 1) an individualized parent-implemented intervention (PII) offered in 2-3 weekly sessions to teach parents how to embed strategies to support social communication skills for 25 hours a week within everyday routines, activities, and places; and 2) an information, education and support group (IES) offered twice monthly. Each site will recruit 36 new parent-child dyads at 18 months of age, who will be pooled with 18 dyads recruited for a preliminary ongoing study, totaling 54 dyads at each site, randomly assigned to either PII or IES as the first condition. Using a crossover design, all dyads will also receive the other condition beginning at 27 months of age, totaling 18 months of intervention. The specific aims of this investigation are: 1) to compare the effectiveness of PII and IES at 27 months; and 2) to compare the effectiveness of the combination of PII and IES at 36 months. Outcome measures of social communication and autism symptoms will be gathered bi-monthly from 18 to 36 months of age and measures of developmental levels and adaptive behavior will be gathered before treatment at 18 months of age, at the point of crossover at 27 months, and after the 2nd treatment condition at 36 months. Hierarchical linear modeling will be used on the combined sample of 108 dyads to estimate growth curves of the outcome measures and to examine differences in growth trajectories for children during the different treatment conditions and starting intensive treatment at different ages. The findings of this study will: 1) document improved outcomes for very young children with ASD receiving a manualized, community-based, cost efficient, appropriate, and effective intervention; and 2) substantiate that intensive intervention beginning by 18 months of age leads to better outcomes than intervention in the third year of life, providing evidence that autism screening for toddlers is crucial to effective early intervention.